Fix: Check always for skb in mptcp_is_available

mptcp_is_available may get called without an skb - Thus we have to be
sure that it exists.

Fix provided by Per Hurtig.

Reported-by: Per Hurtig <per.hurtig@kau.se>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Correctly support the B-flag in the MP_CAPABLE

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Correctly handle DATA_FIN

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Correctly handle malformed DSS-mappings

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Don't account subflow-fin when checking for valid mapping

The subflow-fin is not part of the DSS-mapping. Thus, when calculating
the end_seq of the TCP-space during mapping-checks, we have to take this
into account.

Reported-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Update version-number

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>